Each working cycle of a four-stroke engine is composed of an intake stroke, a compression stroke, a working stroke, and an exhaust stroke. For a four-stroke engine, in order to complete a working cycle, the piston in the cylinder needs to travel four times back and forth (i.e., the crankshaft turns twice). The four-stroke engine is further divided into four-stroke gasoline engine and four-stroke diesel engine. The main difference between these two kinds of the four-stroke engines is the ignition mode. The gasoline engine uses a spark plug ignition, while the diesel engine uses a compression ignition.
The four-stroke engine belongs to the reciprocating piston internal combustion engine, which can be divided into three types—a gasoline engine, a diesel engine, and a gaseous fuel engine according to different types of fuel being used. The reciprocating piston internal combustion engine which uses gasoline or diesel as fuel is called the gasoline engine and the diesel engine respectively. The reciprocating piston internal combustion engines that use natural gas, liquefied petroleum gas, and other gaseous fuel are called the gaseous fuel engine. Gasoline and diesel are both petroleum products, and are traditional fuels for the automobile engine. Non-petroleum fuel is called substitute fuel. The engine which uses the substitute fuel is called substitute fuel engine, such as the ethanol engine, the hydrogen engine, the methanol engine etc.
In view of the heat balance of the current automobile engine, the power for power outputting generally accounts for only 30%-45% (diesel engine) or 20%-30% (gasoline engine) of the total heat of fuel combustion. The power which is discharged as residual heat out of the automobile accounts for 55%-70% (diesel engine) or 70%-80% (gasoline engine), mainly includes the heat taken away by recirculating cooling water and the heat taken away by exhaust gas. The following table 1 is a heat balance table of internal combustion engine.
TABLE 1Heat balance table of internal combustion engineitems of thermalgasolinehigh-speed dieselmiddle-speed dieselbalance %engineengineengineheat of heat balance20-3030-4035-45of effective workheat taken away by25-3020-2510-20coolantheat taken away by40-4535-4030-40exhaust gasother heat loss 5-10 5-1010-15The gasoline engine makes good mixture gas by mixing air with gasoline at a certain ratio. The mixture gas is inhaled into a cylinder during the intake stroke. Then the mixture gas is compressed and ignited to burn to generate thermal energy. The gas with high temperature and high pressure acts on the top of the piston to push the piston to perform reciprocating linear motion, outputting mechanical energy externally through a connecting rod, a crankshaft, and a flywheel. The four-stroke gasoline engine accomplishes an intake stroke, a compression stroke, a working stroke, and an exhaust stroke within a working cycle.
Intake stroke: the piston is driven by the crankshaft to move from the top dead center (TDC) to the bottom dead center (BDC). An intake valve opens at this moment, an exhaust valve is closed, and a crankshaft rotates 180°. During the movement of the piston, the volume of the cylinder is gradually increased. The pressure of the gas in the cylinder is gradually decreased from pr to pa to form a certain vacuum degree. Mixture gas of air and gasoline is inhaled into the cylinder through the intake valve, and is further mixed in the cylinder to form a combustible mixture gas. Since the intake system has resistance, at the intake end point (point A in the figure), the pressure of the gas in the cylinder is less than atmospheric pressure 0p, that is, pa=(0.80-0.90) 0p. The temperature of the combustible mixture gas entered into the cylinder is increased to 340-400K, because the combustible mixture gas is heated by high-temperature parts such as the intake pipe, the cylinder wall, the piston head, valves, the combustion chamber wall and etc., and is mixed with the residual exhaust gas.
Compression stroke: in the compression stroke, the intake valve and the exhaust valve are both closed. The piston moves from the BDC to the TDC, and the crankshaft rotates 180°. When the piston is moving upward, the working volume is reduced gradually. The pressure and the temperature of the mixture gas in the cylinder are constantly increased after the compression. When the compression end point is reached, the pressure pc of the mixture gas can reach 800-2000 kPa, and the temperature of the mixture gas reach 600-750K. In the indicator diagram, the curves of the compression stroke are curves a-c.
Working stroke: when the piston approaches TDC, the combustible mixture gas is ignited by the spark plug. The combustible mixture gas burns and releases a lot of heat, causing the pressure and temperature of the gas in the cylinder to increase rapidly. The maximum combustion pressure pZ reaches 3000-6000 kPa, and the temperature TZ reaches 2200-2800K. The gas with high temperature and high pressure pushes the piston to move from the TDC to the BDC, and outputs the mechanical energy externally through a crank and connecting rod mechanism. With the piston moving downward, the volume of the cylinder is increased, and the pressure and temperature of the gas are reduced gradually. When b point is reached, the pressure of the gas is reduced to 300-500 kPa, and the temperature is reduced to 1200-1500K. In the working stroke, the intake valve and the exhaust valve are both closed, and the crankshaft rotates 180°. In the indicator diagram, the curves of the working stroke are curves c-Z-b.
Exhaust stroke: in the exhaust stroke, the exhaust valve is open, and the intake valve is still closed. The piston moves from the BDC to the TDC, and the crankshaft rotates 180°. When the exhaust valve is open, on one hand, the burned exhaust gas is discharged out of the cylinder under the effect of the cylinder pressure difference between inside and outside. On the other hand, the burned exhaust gas is discharged out of the cylinder by the pushing-out effect of the piston. Due to the resistant effect of the exhaust system, the pressure at the exhaust end point, r point, is slightly more than the atmospheric pressure, that is, pr=(1.05-1.20) p0. The temperature of the exhaust end point is Tr=900-1100K. When the piston comes to the TDC, a certain volume of exhaust gas is left and cannot be discharged. This part of the exhaust gas is called residual exhaust gas.
The four-stroke diesel engine is similar to the gasoline engine. Each working cycle is also composed of an intake stroke, a compression stroke, a working stroke, and an exhaust stroke. Since the diesel engine uses diesel as fuel, compared with the gasoline, the diesel has a low self-ignition temperature, large viscosity, and is hard to volatilize. The diesel engine uses compression end point self-ignition. The working process and system structure of the diesel engine are different from those of the gasoline engine.
Intake stroke: the working medium which enters the cylinder is pure air. Since the resistance of the intake system of the diesel engine is small, the pressure of the intake end point is pa=(0.85-0.95) p0, which is higher than that of the gasoline engine. The temperature of the intake end point is Ta=300-340K, which is lower than that of the gasoline engine.
Compression stroke: since the compressed working medium is pure air, the compression ratio of the diesel engine is higher than that of the gasoline engine (generally, ε=16-22). The pressure of the compression end point is 3000-5000 kPa. The temperature of the compression end point is 750-1000K, which is greatly more than the self-ignition temperature of the diesel (about 520 K).
Working stroke: when the compression stroke is approaching the end, under the effect of the high pressure oil pump, the diesel is injected with a high pressure of about 10 Mpa to the combustion chamber of the cylinder via a fuel injector. Upon being mixed with the air in a short time, the diesel self-ignites and burns immediately. In the cylinder, the pressure of the gas increases rapidly, reaching up to 5000-9000 kPa. The highest temperature is 1800-2000K. Since the diesel engine self-ignites and burns under compression, the diesel engine is called compression ignition engine.
Exhaust stroke: the exhaust of the diesel engine is basically the same as that of the gasoline engine, only that the exhaust temperature is lower than that of the gasoline engine, generally, Tr=700-900K. As for the single-cylinder engine, the rotational speed is nonuniform, the working of the engine is unstable, and the vibration is severe. That is because only one stroke out of the four strokes is working, while the other three strokes are the strokes that consume power to prepare for working. To solve this problem, the flywheel must have sufficiently rotational inertia, which will lead to the increase of the weight and size of the whole engine. Using multi-cylinder engine can offset the above deficiency. Modern automobiles usually use the four-cylinder engine, the six-cylinder engine, and the eight-cylinder engine.
After the cylinder in the above internal combustion engine works, the temperature in the cylinder reaches above 1000K. The high temperature gas is discharged through the exhaust valve, leading to the direct waste of the thermal energy. The temperatures of parts like the inner wall of the cylinder, the piston head, the cylinder head, the valves, etc. are high, which will affect the efficiency of compression stroke. Thus, the cooling system is provided on all the cylinders of the existing engine.
Regarding the utilization of the exhaust gas of the engine, current engines may have a turbo booster. After boosting, the pressure and temperature of the engine are significantly increased during the working. Therefore, the lifetime of the engine will be shorter than that of the engine which has the identical emission without boosting. Furthermore, the mechanical performance and the lubrication performance are both affected. Thus, the application of the turbo boost technology in the engine is limited in a certain degree.
The utilization of the waste heat of the exhaust gas is low. The energy recovery device should resist to the vibration and the shock. The waste heat recovery device of the exhaust gas cannot affect the normal working performance of the engine. Currently, there are several following main methods of using the exhaust gas waste heat of the engine. An exhaust turbocharge uses a part of the energy of the exhaust gas to improve the intake pressure of the internal combustion engine to increase the volume of gas, to improve the power property and economy of the engine. Currently, quite a few automobiles use the method of the turbo boost. However, the method of the turbo boost can only employ a part of the energy of the exhaust gas. Furthermore, there are some problems, for example, the whole working condition of the engine is hard to match, etc.
There are three methods of using the exhaust gas of the engine to generate power, i.e., the thermoelectric power generation, the exhaust gas turbine power generation, and the Polytetrafluoroethylene(PTFE) turbine power generation. The thermoelectric power generation mainly uses thermoelectric power generation material to generate power. However, since the energy conversion rate of thermoelectric material is low, it is required that the thermoelectric conversion material with a high energy conversion rate should be developed. The exhaust gas turbine power generation uses the exhaust gas to drive the turbine to make the generator to generate power. This method of power generation has certain influences on the performance of the engine, which needs further study.
Currently, methods of refrigeration using the waste heat of the exhaust gas of the engine mainly are the absorption refrigeration and the sorption refrigeration. The principle of the absorption refrigeration is that the heat is used as power to accomplish the refrigeration cycle. The sorption refrigeration uses properties of some solid materials, which can absorb a certain gas or steam at a certain temperature and pressure, and can release the gas or steam at another temperature and pressure, to realize refrigeration. According to the present situation of the utilization of the waste heat of the exhaust gas of the engine at home and abroad, a concept of the method of using the waste heat of the exhaust gas of the engine to generate heat and power is provided.
The basic structure of the single-cylinder engine includes a cylinder, a piston, a connecting rod, a crankshaft, a cylinder head, a body, a camshaft, an intake valve, an exhaust valve, a valve spring, and a crankshaft toothed pulley, etc. The working chamber of the reciprocating piston engine is called a cylinder. The internal surface of the cylinder is cylindrical. The piston that performs reciprocating movement in the cylinder is hinged with one end of the connecting rod through a piston pin. The other end of the connecting rod is connected to the crankshaft to form the crank and connecting rod mechanism. When the piston performs reciprocating movements in the cylinder, the crankshaft is pushed to rotate by the connecting rod, or vice versa. At the same time, the volume of the cylinder is continuously changed from small to large, and then from large to small. The cycle is repeated continuously. The top of the cylinder is sealed with the cylinder head. The intake valve and the exhaust valve are mounted on the cylinder head. Through the opening and closing of the intake valve and the exhaust valve, the inflating of the gas into the cylinder and the discharging of the exhaust gas from the cylinder can be achieved. The opening and closing of the intake valve and the exhaust valve are driven by the camshaft. The camshaft is driven by the crankshaft through a toothed belt or a gear. The part which forms the cylinder is called the cylinder body. The crankshaft rotates in the crankcase.
Since the working medium does not burn, the external combustion engine avoids the knocking problem of working of the traditional internal combustion engine. Thus, the external combustion engine achieves a high efficiency, a low noise, a low pollution, and a low running cost. Once the heat chamber reaches 700° C., the apparatus can work and run immediately. The lower the environmental temperature is, the higher the efficiency of power generation is. The most remarkable advantage of the external combustion engine is the output and efficiency are not limited by the altitude, which is suitable to be used in high altitude areas.
At the same time, the main existing problems and defects of the Stirling Engine are as follows. The manufacture cost is high. The sealing technology of working medium is difficult. The reliability and service life of the seal components have problems. The cost of the material is high. The power adjusting and controlling system is complicated. The machine is relatively bulky. The costs of the expansion chamber, the compression chamber, the heater, the cooling chamber, the regenerator, etc. are high. The heat loss is 2-3 times that of the internal combustion engine, etc.
Organic Rankine Cycle system includes a pump, an evaporator, an expander, a generator, a condenser, etc. The heat collector absorbs the solar irradiance, and the temperature of heat exchange medium in the heat collector is increased. The heat exchange medium transfers the heat to the organic medium through the evaporator. The organic medium is heated in the evaporator under a constant pressure. The gaseous organic medium with high pressure enters the expander to work to drive the generator to generate power. The organic medium which is discharged from the end of the expander enters the condenser to condense under a constant pressure. The organic medium from the outlet of the condenser enters the evaporator after being compressed by the pump, to accomplish a generation cycle.
Organic Rankine Cycle system has problems of low conversion efficiency, large volume, and doing work by means of an expander which has a complex structure.
The existing engine, especially, the multi-cylinder engine with a large emission, is noisy.